System and method for predictive link planning

ABSTRACT

A system and method for predictive adaptive coding and modulation (ACM) is disclosed. Predictive ACM is used between a transmitting terminal that is in motion and a stationary receiving terminal without a return link between the terminals where the geometry and link impairments are known in advance. A system and method for predictive ACM in a system including one or more relay terminals between the transmitting and receiving terminals, where the geometry and link impairments between all of the terminals are known in advance is also disclosed.

BACKGROUND

The invention relates generally to adaptive coding and modulation (ACM)and more particularly to ACM in a transmitter without the use of linkparameters provided over a return link from a receiver in acommunications system.

Adaptive coding and modulation (ACM), also known as link adaptation, isused in wireless communication systems to coordinate transmissionsbetween transmitters and receivers. ACM involves adjusting variousaspects of data transmission such as the type of modulation coding orbit error rates in accordance with information about the radio link, orchannel. Channel state information (CSI) is commonly received via areturn link from a receiver, forming a closed loop system. The receiverprimarily provides information about how channel conditions arechanging.

A representative prior art system 10 is shown in FIG. 1. At thebeginning of a transmission, i.e. when a link is initially established,a transmitting terminal 12 chooses initial link parameters blindly,without a priori information, and begins transmission 14 to receivingterminal 16. Receiving terminal 16 receives transmission 14 andgenerates CSI, for example, an estimate of the signal to noise ratio(SNR) or bit error rate (BER) of the received signal. These are sentover return link 18 back to transmitting terminal 12. In response,transmitting terminal 12 adapts link parameters for subsequenttransmissions based on the received CSI. Link metrics are sent at a ratethat is commensurate with the rate of change for the channel ofinterest, with values typically ranging from seconds to tens of seconds.

Unfortunately, in many communications systems, providing a return linkis challenging or impractical. Two such links are provided as examples.The first is an application in which a communications link is onedirectional, such as when the transmitting terminal is a telemetry orsensor link. In this application adding return link hardware, comprisedof an additional receiver, low-noise amplifier, and diplexer to thetransmitting terminal, solely for the purpose of receiving return linkmetrics, may be impractical due to the additional cost, size, weight,and power required. A second application where a return link ischallenging or impractical is one in which a link is designed tominimize the probability of detecting the signal being transmitted. Inthis application, adding a secondary link in the opposite direction torelay CSI could conceivably double the chances of the signal beingdetected, which is an undesirable tradeoff.

Thus, a need exists for predictive link planning from a transmittingterminal and to a receiving terminal without a return link from thereceiving terminal to the transmitting terminal. In addition, a needexists for ACM and predictive link planning in a system in which thegeometry and link impairments between two or more communicatingterminals can be predicted ahead of time.

SUMMARY

The invention in one implementation encompasses a system and method forpredictive adaptive coding and modulation (ACM) between a transmittingterminal and a receiving terminal without a return link between theterminals where the geometry and impairments of the link are known inadvance. There is a further need for predictive ACM in a systemincluding one or more relay terminals between the transmitting andreceiving terminals, where the geometry and impairments of the linksbetween all of the terminals are known in advance.

In an embodiment, the invention encompasses a method of transmittingdata from a transmitting terminal to a receiving terminal over achannel, the method including steps of determining a series of locationsfor each of the transmitting and receiving terminals; determining a linkgeometry of the channel between the transmitting terminal and thereceiving terminal for each location in the series of locations;determining channel impairments for the link geometries; predictingsignal-to-noise ratios (SNRs) of the channel for the link geometries andchannel impairments; storing channel parameters based on the predictedSNRs in a lookup table; and transmitting data from the transmittingterminal using channel parameters retrieved from the lookup table.

In a further embodiment, the step of determining a link geometry furtherincludes determining a distance and associated pointing angles betweenthe transmitting and receiving terminals for each location in the seriesof locations.

In any of the above embodiments, the channel parameters are retrievedfrom the lookup table using a location of the transmitting terminal.

In any of the above embodiments, the channel parameters are retrievedfrom the lookup table using a time elapsed since a previous access tothe lookup table.

In any of the above embodiments, the channel parameters further includeat least one of a channel symbol rate, modulation type, code rate, codetype or frequency.

In yet another embodiment, the channel includes one or more relayterminals and the step of determining a series of locations furtherinclude determining locations of the one or more relay terminals.

In the above embodiments, the step of determining a link geometry of thechannel includes determining a first link geometry between thetransmitting terminal and the one or more relay terminals and a secondlink geometry between the one or more relay terminals and the receivingterminal, for each location in the series of locations.

In another embodiment, before the step of determining a series oflocations, a step of using elapsed time or location to select thereceiving terminal from a set of receiving terminals based on a prioriknowledge of link geometry and an estimate of the current location ofthe transmitting terminal is performed.

In any of the above embodiments, the receiving terminal detects changesin the transmitting data and automatically adapts to the modulation andcoding selected by the transmitting terminal.

In any of the above embodiments, the lookup table further includes aplurality of lookup tables for different frequencies or times of year.

In another embodiment, the invention encompasses communication systemfor providing predictive adaptive coding and modulation (ACM) duringtransmission between terminals, having: one or more receiving terminalsfor detecting changes in a transmission rate and automatically adaptingits demodulation to the changes; and a transmitting terminal fortransmitting data to the one or more receiving terminals usingpredictive ACM by selecting channel parameters from a lookup tablewithout receiving channel parameters over a return link from the one ormore receiving terminals.

In a further embodiment, the lookup table includes channel parametersfor a plurality of locations of the transmitting terminal and isaccessed using a current location of the transmitting terminal.

In another embodiment, the lookup table includes channel parameters fora plurality of locations along a planned trajectory of the transmittingterminal and is accessed using a time elapsed since a previous access ofthe lookup table.

In any of the above embodiments, the lookup table includes a pluralityof lookup tables for different frequencies or times of year.

In any of the above embodiments, the channel parameters further compriseat least one of a channel symbol rate, modulation type, code rate, codetype or frequency.

In any of the above embodiments, the channel parameters are based on apredicted signal-to-noise ratio (SNR).

In another embodiment, the invention encompasses a transmitting terminalin a communication system, the transmitting terminal sending data to areceiving terminal in the communication system using the methoddescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

Features of example implementations of the invention will becomeapparent from the description, the claims, and the accompanying drawingsin which:

FIG. 1 is a block diagram of a prior art transmission system.

FIG. 2 is a flowchart illustrating a method of generating a lookuptable.

FIG. 3 is a transmission system according to the present invention

FIG. 4 shows a first embodiment of the transmission system of FIG. 3.

FIGS. 5A and 5B show a second embodiment of the transmission system ofFIG. 3.

FIG. 6 shows a third embodiment of the transmission system of FIG. 3.

DETAILED DESCRIPTION

Reference will now be made in detail to one or more embodiments of theinvention. While the invention will be described with respect to theseembodiments, it should be understood that the invention is not limitedto any particular embodiment. On the contrary, the invention includesalternatives, modifications, and equivalents as may come within thespirit and scope of the appended claims. Furthermore, in the followingdescription, numerous specific details are set forth to provide athorough understanding of the invention. The invention may be practicedwithout some or all of these specific details. In other instances,well-known structures and principles of operation have not beendescribed in detail to avoid obscuring the invention.

In an embodiment, the invention encompasses a system for wirelesscommunication between a transmitting terminal and one or more receivingterminals. Adaptive coding and modulation (ACM) is used to enhancetransmissions. Rather than performing ACM in response to channel stateinformation (CSI) received from a receiving terminal, the transmittingterminal performs predictive link planning using any or all of thefollowing information:

1. Terminal locations as determined by a navigational device or bymeasuring elapsed time during a planned trajectory.

2. Geometry between two or more terminals, to include distance betweenterminals and associated pointing angles (e.g. azimuth and elevationangles) between the terminals for the communication path(s).

3. Deterministic channel impairments that affect the average SNR betweena transmitting terminal and one or more receiving terminals, asdetermined by link frequency, terminal locations, time of year,anticipated weather, transmitter output back off (OBO) and distortion,and receiver gain over temperature (G/T) performance. Such effectsinclude, but are not limited to, weather loss effects, multipatheffects, terrain blockage effects, error vector magnitude (EVM) effects,relay satellite effects, scintillation and gas loss effects,polarization loss effects, and effects due to additional noise sourcesbased on geometry.

The link parameters to be varied include, for example, modulation type(e.g. BPSK, QPSK, 8PSK, etc.); forward error correction (FEC) code rateand/or code type; symbol rate and bandwidth; and communicationfrequency. These parameters are used as a priori information to maximizethe instantaneous communication link rate between two or more terminals.In a system with more than two terminals, intermediate, or relay,terminals function as both a receiving and a transmitting terminal. Thediscussion below with regard to transmitting and receiving terminalsrefers to the interaction between any pair of terminals.

A method of predictive link planning according to the present inventionis illustrated in FIG. 2. The link between a transmitting terminal andone or more receiving terminals is planned in advance considering theabove factors and using a priori information to generate a lookup tablethat is loaded into the transmitting terminal. The lookup table canspecify link parameters based on transmitting terminal location,receiving terminal location, or both transmitting terminal and receivingterminal locations. In addition or as an alternative, the lookup tablecan specify link parameters based on time when a trajectory of one orboth terminals is known. In yet another alternative, multiple lookuptables can be implemented for different frequencies, times of year, orother relevant variations.

Referring to FIG. 2, a method for generating a look-up table isillustrated. For each point in time at step 20, terminal locations arecalculated based on planned locations or trajectories of one or bothterminals, for example, an orbit or a flight path. The locationsinclude, for example, latitude, longitude or elevation. At step 22, thelink geometry between transmitting and receiving terminals iscalculated. The link geometry includes, for example, a distance betweenterminals and associated pointing angles (e.g. azimuth and elevationangles) although other measurements or characteristics could be used. Atstep 24, the method calculates channel impariments based, for example,on link conditions, link geometry, weather and channel frequencies. Atstep 26, an average channel signal-to-noise-ratio (SNR) is predictedbased on link geometry and channel impairments. At step 28, a channelsymbol rate, modulation type, code rate, code type and frequency is thencalculated for the current point. Finally, at step 29, the methodreturns to step 20 for each point in all planned trajectories.

The points in time at which the steps above are performed are selectedwith a certain frequency. The appropriate frequency depends on the rateof change of the link geometry, which further depends on thetransmitting and receiving terminal mobility, both velocity and heading.If the link is changed too infrequently, the link may drop due tounaccounted for changes in link condition. In an embodiment, terminallocations are updated approximately every 30 to 300 seconds.

A system implementing the look-up table generated by the method of FIG.2 is shown in FIG. 3. Similarly to FIG. 1, transmitting terminal 30sends a transmission 32 to receiving terminal 34. However, receivingterminal 34 does not send link parameters over a return link. Instead,transmitting terminal 30 uses values from lookup table 36 to dynamicallychange the link parameters depending on lookup table values, and thereceiving terminal 34 adapts to the link parameters established bytransmitting terminal 30. At the beginning of a transmission,transmitter 30 determines time or location values that are used to indexlookup table 36, as will be explained in more detail below. Each oftransmitting terminal 30 and receiving terminal 34 include one or moreprocessors (not shown) for executing the operations and steps describedherein. In an embodiment, lookup table 36 is created in this processoror in one or more other processors external to the transmitting andreceiving terminals.

In an embodiment, a method of the invention is performed with noexchange of link metrics between the transmitting and receivingterminals—link adaptation is done entirely based on link performancethat is predicted ahead of time, and the process is entirely open loop,with no return CSI exchanged between the receiving and transmittingterminal. The invention in this method relies on a priori knowledge tooperate without a return link between the transmitting terminal andreceiving terminal, and is suited for applications where a return linkmay be undesirable or impractical. During a transmission, the receivingterminal detects changes in the transmission and automatically adapts tothe modulation and coding selected by the transmitter.

A system and methods according to the invention will be explained inconnection with several embodiments. In a first embodiment a mobiletransmitting terminal is transmitting to a stationary receiving terminalas depicted in FIG. 4. Aircraft 40 is transmitting to ground terminal42. Aircraft 40 is shown at two points in time (t₁ and t₂) as it fliesalong a trajectory. In this example, although the specific trajectory isnot known in advance, the location of the aircraft can be determined atany given time via GPS or a similar navigational device. This allows thelink parameters to be determined based on an estimate of transmittingterminal and receiving terminal geometry to account for the changinglink conditions (e.g. elevation angle, distance, regional weather) in ana priori fashion. Thus, the method of FIG. 2 prepares a lookup table forall anticipated receiver and transmitter geometries and is indexed byrelevant parameters, for example, elevation angle, distance, regionalweather, etc. In other words, the transmitting terminal knows its GPScoordinates, and from these it determines the current geometry of thelink, particularly in terms of elevation angle and distance, althoughother dimensions of the link geometry could be used to index the lookuptable as well.

In an alternative, the method of the first example is used in the casewhere an aircraft is flying a planned trajectory that is known ahead oftime. In this case, elapsed time could be used instead of aircraftlocation as an input to the lookup table used for link parameters.

In a second embodiment, a receiving terminal is mobile and atransmitting terminal is either mobile or stationary. In thisembodiment, the transmitting terminal, if mobile, knows its locationeither by elapsed time or by a navigational device. However, thelocation of the receiving terminal must be determined by thetransmitting terminal using elapsed time for a known trajectory.

A third embodiment of the invention is described in connection withFIGS. 5A and 5B. This embodiment features multiple receivers andtransmitters, and is comprised of, for example, an orbiting satellite 50transmitting to a ground station 54 through an orbiting relay satellite52. While ground station 54 is stationary, both satellites 50 and 52 aremobile. Representative locations, and therefore overall link geometry,at a time t₁ are shown in FIG. 5A, and representative locations are atime t₂ are shown in FIG. 5B. Since the satellites have fixed and knownorbits, the exact location of both satellites 50 and 52 at any giventime is known a priori. This allows a lookup table to be constructedthat varies the link parameters to accommodate the changes in linkgeometry for link 51 between satellites 50 and 52, as well as thechanges in link geometry for link 53 between the relay satellite 52 andthe ground station 54 entirely on information that is known beforehand.In this case, the lookup table would be indexed based on elapsed time,and the geometry of each link 51 and 53 is used to estimate signal tonoise ratio (SNR) via a separate lookup table, with the composite linkSNR, given by the equation

${{SNR}_{Composite} = {\frac{1}{{SNR}_{1}} + \frac{1}{{SNR}_{2}}}},$where SNR₁ is for link 51 and SNR₂ for link 53, and all equation termsare linear. As described above, the transmitting terminal determines thechannel SNR for the entire channel all the way to ground station 54 andrelay satellite 52 simply receives and retransmits the signal withoutdemodulation without changing any signal parameters of interest such asmodulation type, code rate, symbol rate, etc. In an alternativeembodiment, relay satellite 52 functions as both a receiving terminaland a transmitting terminal. In this alternative embodiment, the entirechannel is essentially a 2-hop (or more) relay link where transmittingsatellite 50 and relay satellite 52 form a transmit-receive pair, as dorelay satellite 52 and ground station 54. Thus, relay satellite 52 woulduse its own lookup table when transmitting similar to that describedabove for the transmitting terminal.

A fourth embodiment of the invention is described in connection withFIG. 6. This embodiment features a single satellite transmitter 60 andmultiple ground station receivers 62 and 64. Since the satellite 60 hasfixed and known orbits, and the location of each ground receiver 62, 64is fixed, the geometry to each of the ground stations can be determineda priori. In this embodiment multiple lookup tables for each groundstation can be constructed, and for each point in time the optimalground station that maximizes the communication rate can be selectedbased on a priori link calculations, achieving an optimal link not onlyto each ground station, but among multiple ground stations without theuse of return link CSI.

Although specific embodiments have been discussed in connection withFIGS. 4-6, these are representative for the purposes of explaining theinvention. Any of the transmitting terminals, receiving terminals, orrelays may be mobile or stationary. A mobile terminal or relay may be,for example, a satellite or a manned or unmanned aircraft. Atransmitting terminal may be mobile or stationary, with its locationdetermined either by a navigational device or elapsed time for a knowntrajectory.

In the embodiments of FIGS. 5 and 6, the relay between a sourcetransmitting terminal and a destination receiving terminal may be eithermobile or stationary, but the location of the relay must be determinedby the source transmitting terminal using elapsed time for a knowntrajectory. Further, a system according to the present invention mayinclude more than one relay.

A destination receiving terminal may also be mobile or stationary butthe location of the destination receiving terminal must be determined bythe source transmitting terminal using elapsed time for a knowntrajectory.

Numerous alternative implementations of the present invention exist. Forexample, other applications include a race car telemetry link for a carfollowing a known trajectory around a race course, or commercialshipping applications in which an aircraft or ship navigates along wellestablished routes, or railway systems that are constrained tonavigation along predetermined track locations. Although notspecifically depicted, two or more relay terminals may be used in asystem according to the present invention.

The steps or operations described herein are just for example. There maybe many variations to these steps or operations without departing fromthe spirit of the invention. For instance, the steps may be performed ina differing order, or steps may be added, deleted, or modified.

Although example implementations of the invention have been depicted anddescribed in detail herein, it will be apparent to those skilled in therelevant art that various modifications, additions, substitutions, andthe like can be made without departing from the spirit of the inventionand these are therefore considered to be within the scope of theinvention as defined in the following claims.

What is claimed is:
 1. A method of transmitting data from a transmittingterminal to a receiving terminal over a channel, comprising the stepsof: determining a series of locations for each of the receivingterminals and for exactly one transmitting terminal; determining a linkgeometry of the channel between the transmitting terminal and thereceiving terminal for each location in the series of locations, whereindetermining the link geometry comprises determining a distance betweenthe transmitting and receiving terminals for each location in the seriesof locations; determining channel impairments for the link geometries byvarying link parameters; using the link parameters as a prioriinformation to maximize an instantaneous communication link rate betweenthe receiving terminal and the transmitting terminal; predictingsignal-to-noise ratios (SNRs) of the channel for the link geometries andthe channel impairments; storing channel parameters based on thepredicted SNRs in a lookup table; and transmitting data from thetransmitting terminal using channel parameters retrieved from the lookuptable, wherein the channel parameters are retrieved from the lookuptable using the distance between the transmitting and receivingterminals.
 2. The method of claim 1, wherein the step of determining alink geometry further comprises determining associated pointing anglesbetween the transmitting and receiving terminals for each location inthe series of locations.
 3. The method of claim 1, wherein the channelparameters further comprise at least one of a channel symbol rate,modulation type, code rate, code type or frequency.
 4. The method ofclaim 1, wherein the channel further comprises one or more relayterminals and the step of determining a series of locations furthercomprises determining locations of the one or more relay terminals. 5.The method of claim 4, wherein the step of determining a link geometryof the channel comprises determining a first link geometry between thetransmitting terminal and the one or more relay terminals and a secondlink geometry between the one or more relay terminals and the receivingterminal, for each location in the series of locations.
 6. The method ofclaim 1, further comprising, before the step of determining a series oflocations, a step of using elapsed time or location to select thereceiving terminal from a set of receiving terminals based on a prioriknowledge of link geometry and an estimate of the current location ofthe transmitting terminal.
 7. The method of claim 1, wherein thereceiving terminal detects changes in the transmitting data andautomatically adapts to the modulation and coding selected by thetransmitting terminal.
 8. The method of claim 1, wherein the lookuptable further comprises a plurality of lookup tables for differentfrequencies or times of year.
 9. A transmitting terminal in acommunication system, said transmitting terminal sending data to areceiving terminal in the communication system using the method ofclaim
 1. 10. The communication system of claim 1, wherein the linkparameters comprise one or more of a modulation type, a forward errorcorrection (FEC) code rate and/or code type; symbol rate and bandwidth;and communication frequency.